1. Field of the Invention
The present invention relates to the damping of pressure pulsations in a fluid system. More particularly, the present invention relates to a gas charged pulsation damping assembly for damping pressure pulsations in power, transmission or control systems.
2. Setting of the Invention
Working fluid used in power, transmission and control systems may be subjected to periodic, rapid pressure increases and decreases. As used herein, the term xe2x80x9cworking fluidxe2x80x9d is intended to refer to liquids and gases, both flowing and static, used to monitor, power or regulate machinery or to the fluids moving through a pipeline, or to other fluids that are the effective or active fluids in a static or dynamic system.
These pressure fluctuations in working fluids, referred to generally as xe2x80x9cpulsations,xe2x80x9d can damage and interfere with the operation of the systems. Pressure pulsations are frequently induced by positive displacement pumps associated with systems. A wide variety of devices have been developed to dampen the pulsations. A common pulsation damping technique allows the working fluid pressure to be exerted against an energy-absorbing device that tends to diminish the amplitude of the pulsations.
A common xe2x80x9ctubularxe2x80x9d design used for pulsation damping of flowing fluids employs a perforated tube section extending centrally through an annular, gas-pressurized diaphragm contained within a section of a system fluid flow line. One such design is illustrated in U.S. Pat. No. 4,759,387. Pulsations in the working fluid (usually a liquid) flowing through the perforated tube are damped by distending the diaphragm radially outwardly. This action forces the working fluid within the tube to flow radially through the perforations in the tube, thereby dissipating a portion of the pulsation energy with no system loss of flow or drop in pressure of the working fluid.
Pulsation damping systems using the described prior art design are usually intended to operate within a relatively narrow pressure range within which the diaphragm is substantially unstressed. If the system is operated outside its optimum pressure range, the pressurizing gas and the pressure surges caused by pulsations in the working fluid can stretch and thereby stress the resilient diaphragm. By virtue of their design, these prior art pulsation damping systems are relatively large and the performance and efficiency of such systems vary as a function of the gas pressure charges acting against the diaphragm. One prior art pulsation damping arrangement of the described xe2x80x9ctubularxe2x80x9d design exhibits a significant decrease in damping ratio as the gas charge (determined as a percentage of working fluid pressure) is increased toward the operating pressure of the working fluid.
These xe2x80x9ctubularxe2x80x9d pulsation damping systems require the fabrication of a perforated tubular section that underlies and supports the inflatable diaphragm. Some of such systems require additional structural supporting materials to prevent the perforated tube from collapsing under the compressive force exerted by the pressurized diaphragm. These perforated tubes and associated structural supporting members can be large and expensive to fabricate, particularly when it is necessary to use exotic metals and alloys and other corrosion resistant or specialty strength-enhancing materials.
Another prior art pulsation damping design uses a bellows or a piston-cylinder arrangement disposed in a surge chamber that communicates with the working fluid being damped. U.S. Pat. No. 5,205,326 illustrates pulsation dampers of this type. The bellows compresses, or the piston is driven into the cylinder, as the pressure of the working fluid in the surge chamber increases during pulsation. Compressed gas or mechanical springs are used to resist the compressive force exerted by the fluid pulsation. As with the tubular pulsation damping systems, the damping efficiency of the bellows and piston-cylinder arrangements varies over the range of the internal gas charge or spring force exerted against the pressure responsive element. The bellows and piston-cylinder members of these prior art systems require relatively large components and are also expensive to fabricate.
The prior art also teaches a pulsation damper design employing a relatively thick, flexible diaphragm that rests against a domed, perforated support when the gas pressure charging the diaphragm is sufficiently greater than that of the pressure of the working fluid. The design protects the diaphragm from rupturing when a large pressure differential exists between the pressure of the working fluid and the gas charge pressure. An example of such a design may be seen in U.S. Pat. No. 2,563,257, which employs a perforated plate having a cup or dish shape to support the gas charged diaphragm. The diaphragm is movable in a large chamber between its extreme pressured and unpressured positions without stretching or stressing the diaphragm. Because the diaphragm can move its entire length in either direction from its central mounting point, the pulsation absorber described in the patent requires a chamber that is substantially twice the unstressed axial displacement height of the diaphragm. The cup or dish shape is said in the patent to be preferable to a flat perforated plate in that it acts as an arch and provides a greater area for perforations.
As with the previously described pulsation damping systems employing tubular diaphragms and bellows or piston-cylinder arrangements, the dome-shaped cup or dish design can be relatively large and expensive to fabricate, particularly when it must be constructed of metal alloys or other specialty, strength-enhancing materials. The damping efficiency of the systems can also be widely variable over the range of the pressure variations in the working fluid.
A flexible diaphragm is disposed between a working fluid and a gas-charged chamber to form a pulsation damper. The diaphragm has a flat base that, when fully distended by pressure in the gas chamber, lies against a flat, perforated circular metal sheet. The perforated metal sheet is closely spaced from a planar backing surface that prevents the sheet from permanently deforming under the force exerted by the pressurized diaphragm. An annular channel formed in the backing surface places the working fluid in contact with the perforated metal sheet and permits fluid flow through the assembly when the perforated sheet is engaging the planar backing surface. Pressure pulsations in the working fluid displace the diaphragm away from the perforated metal sheet. A flat retaining wall in the gas chamber limits the travel of the diaphragm away from the metal sheet. Back and forth flow of the pulsing working fluid through the perforations and the compression of the gas in the gas chamber dissipate the energy of the pulses to achieve the damping effect.
The diaphragm is maintained in a non-stressed condition during its movement between the flat metal sheet and the flat retaining wall. The diaphragm moves only in a single direction from its mounting within the body of the damper assembly, which reduces wear of the diaphragm and contributes to reducing the total height of the damping assembly. The lateral walls of the cup-shaped diaphragm are relatively thin compared to the diaphragm base. The thin wall construction enhances the response of the diaphragm to pressure fluctuations in the working fluid while the thicker base protects the diaphragm from damage caused by engagement with, and movement over, the perforated disk.
The design of the components of the present invention coupled with the limited movement, non-stressed operating range of the diaphragm produce a low cost, long-lived assembly that exhibits a linear relationship between its damping ratio verses the gas chamber charge as a percentage of the working fluid pressure, even at percentages approaching 100 percent of the working fluid pressure.
The components of a pulsation damper of the present invention may be easily and inexpensively fabricated from readily available materials. The use of planar surfaces for the perforated metal sheet, the backing surface behind the sheet and the retaining wall in the gas chamber reduces fabrication costs of both the retaining members and the conforming diaphragm. The flat backing surface behind the perforated metal sheet is easily machined or milled to provide a desired fluid course that is maintained in close contact with the sheet. The limited displacement and absence of stress in the diaphragm, over the full operating range of the system, combined with the system design, enables long-lived, efficient operation even at gas pressure charges approaching the design operating pressure of the working fluid.
The design of the pressure damper of the present invention also permits fabrication of a relatively small assembly that can be easily associated with control elements in pressure regulating systems.
The improved operating efficiency and small size of the pressure damping assembly of the present invention enhances its suitability for use in pressure sensitive control systems and other pressure sensitive devices, such as pilot operated pressure relief valves. Pilot operated pressure relief valves are set to open and closed automatically as required to maintain operating pressure levels by relieving excess system pressure. The xe2x80x9cset pressurexe2x80x9d of the relief valve is typically set at some percentage below the maximum allowable working pressure of the piping and equipment associated with the relief valve.
In many commercial systems, such as may be found for example at a chemical plant, it is desirable to operate a xe2x80x9cprocessxe2x80x9d at as high a pressure as possible, within safe operating levels of the piping and associated equipment. Operating at higher pressures permits higher efficiencies and better yields from the process. A pilot operated pressure relief valve protects the piping and attached equipment by relieving excess pressure in the system once the xe2x80x9cset pointxe2x80x9d of the pilot has been exceeded. Without damping, the peaks of the pressure pulsations in the working fluid are typically high enough to activate the pilot and pressure relief valve and/or cause excessive wear in the pilot. To keep from activating the relief valve and reducing the life of the pilot, the plants must reduce the pressure of the process, thus reducing efficiency and yield. Adding a pulsation damper of the present invention allows the process working pressure to be raised closer to the set pressure of the relief valve. The pulsation damper of the present invention achieves a high degree of damping efficiency at gas charge pressures approaching 100 percent of the process working pressure.
In the method of the present invention, multiple pulsation dampers are simultaneously exposed to the working fluid. The pulsation dampers are provided with different gas charges to more effectively dampen pulsations in wider pressure ranges of the working fluid.
In view of the foregoing, it will be appreciated that an important object of the present invention is to provide an efficient pulsation damper having relatively small dimensions, which is inexpensively fabricated from readily available materials.
An important object of the present invention is to provide a small-bodied pulsation damper for damping pressure pulses in a working fluid wherein the pulsation damper exhibits an increasing damping ratio as the gas charge pressure in the damper approaches the pressure of the fluid being damped.
An object of the present invention is to provide an assembly to dampen the pulsations in a monitored fluid in which the damping ratio of the assembly increases linearly as the gas charge in the damper approaches the pressure of the monitored fluid.
A related object of the present invention is to provide an apparatus for damping pressure pulsations in a monitored fluid wherein the damping ratio of the apparatus increases linearly over a range of gas charges that extends beyond a gas charge of 90 percent of the monitored fluid pressure.
Yet another object of the present invention is to provide a small, inexpensively fabricated, damping apparatus that may be used in a pressure sensitive system whereby operation of the pressure sensitive system is optimized.
A specific object of the present invention is to provide a small, inexpensively fabricated, efficient damping apparatus that may be used with a pilot operated pressure relief valve to permit improved efficiency and yield of the process system being protected by the pressure relief valve.